1. Field of the Invention
This invention relates to methods and apparatus for cracking hydrocarbonaceous feedstock, and more particularly to such methods and apparatus as utilize a powdered or granular catalyst in a fluidized state in order to achieve cracking of the feedstock.
2. The Present State of the Art
Fluidization of a powdered or granular material is the process by which the small solid particles of that material are caused to behave collectively like a fluid due to interaction with a fluidization medium, such as a gas or a liquid. If such fluidization mediums are passed upwardly through a bed of fine particles, the resulting frictional forces arising between the fluidization medium and the particles tend to counterbalance the weight of the particles. At a sufficiently high flow velocity in the fluidization medium the particles become suspended therein and the mixture behaves like a fluid.
In the field of petroleum refining, it is common to employ catalysts to stimulate the cracking of the large hydrocarbon molecules in heavy petroleum feedstock into smaller molecules from which conventional motor fuels, such as gasoline, jet fuel, kerosene, and diesel fuel, can be produced. The fluidization of such cracking catalysts in powdered or granular form was first attempted commercially in the early 1940s in an effort to efficiently contact heavy petroleum feedstock with small catalyst particles for cracking purposes. The resulting device, termed a fluid catalytic cracker, now occupies the heart of large modern petroleum refineries.
Since its beginning considerable changes have been made in fluid catalytic cracker technology. The introduction of new catalyst materials and the refinement of mechanical techniques and designs have enhanced the efficiency of the devices. Nevertheless, the mechanical configuration of the fluid catalytic cracker has remained basically the same. That configuration is characterized by massive, tall catalyst reservoir vessels supported by corresponding high-profile structural supports.
In a typical cracker one of the catalyst reservoirs functions as a reactor where the hydrocarbonaceous feedstock is actually cracked by its contact with a fluidized catalyst in a high temperature environment. In this process quantities of carbon by-product, or coke, are gradually deposited on the catalyst particles, rendering these progressively less effective for inducing cracking. Accordingly, a second catalyst reservoir commonly encountered in fluid catalytic cracking units is a regenerator. There coke deposited on the surface of the catalyst particles is burned off using an oxidizing gas, such as air. In this manner used catalyst from the reactor can be continuously recycled as it is used in the cracker. Also involved in a typical fluid catalytic cracker is auxiliary equipment external to the reactor and regenerator that is necessary to fluidize and to advance the solid catalyst, as well as to control the processes of reaction and regeneration.
It is important to maintain a seal between the atmosphere in the regenerator and the atmosphere in the reactor in order to prevent an explosion of the hot hydrocarbonaceous feedstock and cracked hydrocarbon by-products in the reactor. Nevertheless, used catalyst must be transported from the reactor to the regenerator and freshly regenerated catalyst must be supplied from the regenerator to the reactor.
Existing fluid catalytic cracking technology, which utilizes separate vessels for the reactor and regenerator, effects this essential seal through the use of tall reactor and regenerator vessels mounted on structures eighty to two hundred feet in height. In this manner the standing head of catalyst in the vessels and the lines therebetween is used to provide a seal which prevents the oxidizing atmosphere in the regenerator from contacting the hot hydrocarbon materials in the reactor. Typically the regenerator is located higher than the reactor, and a moving catalyst column in the transfer lines therebetween maintains the seal between the atmospheres in each.
The use of tall reactor and regenerator vessels, one or both of which must be supported off the ground, has resulted in modern refining units in which structural costs are a major component. The structural problems involved have been exacerbated by the weight of the large vessels due to the substantial quantities of catalyst to be housed in the vessels.
In many instances feedstock is injected into the catalyst in the transfer lines leading from the regenerator to the reactor. As a result a substantial amount of the cracking that takes place occurs in such riser lines as the mixture of catalyst and feedstock move upwardly prior to actually entering the reactor vessel. Accordingly, the reactor vessel in many instances, rather than serving as the major site of hydrocarbon cracking, functions primarily as a storage container for catalyst which is no longer active in the cracking process.
Continued use in modern refinery technology of huge reactor and regenerator vessels is largely a result of an adherence to the traditional form in which fluid catalytic crackers were first commercially embodied. In addition to causing an inefficient use of catalyst material, the presence of these large storage reservoirs presents other problems, such as the development of poor flow patterns and mixing as the catalyst is advanced through the system.
Ideally, in the reactor, each quantity of feedstock introduced should move through the reactor together with a given quantity of catalyst, which begins as fresh catalyst and after cracking feedstock moves on to the regenerator. This is called idealized plug flow. The cracking capability of catalyst is limited by the degree to which carbon by-product is deposited on the surface thereof. In idealized plug flow the full cracking capacity of a given quantity of catalyst is exhausted on the quantity of hydrocarbon feedstock originally introduced thereinto. Correspondingly, the maintenance of this type of idealized plug flow results in a situation in which newly introduced feedstock in the reactor consistently encounters fresh, rather than used catalyst, and thus cracks the feedstock in an optimally efficient manner.
Large catalyst vessels contain massive volumes of catalyst in which it is virtually impossible to maintain the desirable condition of idealized plug flow. In massive catalyst reservoirs, currents of catalyst commonly flow laterally or even backwards in relation to the predetermined flow direction of the device Both lateral mixing and catalyst backflow result in used catalyst failing to advance properly through the system. As a result, used catalyst tends to be mixed among the fresh catalyst into which new feedstock is introduced. This has the result of reducing the efficiency of the cracking operation.
In addition, in order to effect any flow at all, large, tall catalyst reservoirs of necessity involve high fluidizing pressures and correspondingly high catalyst velocities. This implies the need for expensive and heavy fluidization equipment, such as blowers, piping, and valves exterior to the catalyst vessels themselves. In addition, however, high fluidization pressures and the resulting high catalyst velocities in fluid catalytic crackers have several serious negative ramifications within the transfer lines and the catalyst vessels themselves.
Primary among these is the rapid breakdown or attrition of the catalyst material, as well as the erosion of the catalyst containers due to the abrasive quality of the fluidized catalyst mixture. Rapid catalyst attrition requires the introduction of new catalyst into the system on a regular basis, increasing the costs of operation. Catalyst vessel erosion causes high maintenance costs and substantial system down time to permit servicing and repair.
Conventional fluidized catalytic cracking processes, particularly those involving large catalyst reservoirs, have other drawbacks as well. Far too often these devices provide an overly long residence time during which feedstock and the cracked products thereof remain in the high temperature cracking environment of the reactor. Long residence times result in numerous secondary and undesirable side reactions in the hydrocarbonaceous feedstock. Such severe thermal cracking increases the production of less desirable gaseous products from the feedstock and unnecessarily loads the reactor and catalyst with coke. The result is a decreased yield of desirable condensable hydrocarbon products and used catalyst heavily ladened with coke.
By contrast, it is preferable that volatilized cracked hydrocarbon gases be removed promptly from the reactor vessel in order to minimize the occurrence of such reactions. Protracted residence times are, however, a predictable result of the poor flow and moving characteristics of conventional fluid catalytic crackers which depend on large, tall catalyst reservoirs.